Tetraopes milkweed beetle genomes elucidate the adaptive basis of a temperate coevolutionary radiation

The coevolutionary radiation of 27 species of Tetraopes longhorned beetles and their Asclepias milkweed hosts represents a classic example of adaptive evolution driven by plant chemical defenses and herbivore counteradaptations. Investigations to date, however, have focused on a single species, Tetraopes tetrophthalmus, which feeds on the moderately toxic Asclepias syriaca, providing limited comparative data for hypothesis-testing. Here, we present reference-quality genomes for three Tetraopes species—T. sublaevis, T. tetrophthalmus, and T. melanurus—each largely specialized on an Asclepias host with distinct cardenolide toxicity, to investigate the genomic basis of their adaptations to evolving plant defenses. We identify key amino acid substitutions in the Na+/K+-ATPase α1 associated with cardenolide resistance, with additional changes in species feeding on hosts with more toxic cardenolides. We reveal expansions and relaxed purifying selection on ABC transporter and peritrophic membrane protein genes, with positive selection within T. sublaevis that may facilitate tolerance and sequestration of highly potent nonpolar cardenolides present in Asclepias erosa. Parallel selective pressures on serine endopeptidase genes suggest counteradaptations to root-specific protease inhibitors. In addition, Tetraopes-specific expansions of plant cell wall-degrading enzymes, including GH5_2 hemicellulases and GH28 pectinases, reflect adaptations to digest herbaceous milkweed tissues. These findings illuminate the apparent genomic underpinnings of putative adaptation in Tetraopes beetles to phytochemical and structural defenses of their Asclepias hosts.